Possibly wrong by the drs?

All babies grow at the same rate in the first trimester. So if your sono says 6 weeks 4 d pregnant then that is what you are. They are pretty accurate if you have one early. You also might ovulate late and since sperm can actually live a couple days if the conditions are right then fertilization and implantation could have occurred a couple days after dec 5 even though that was the day you had . Due dates are never exact anyway. My babies are always late and I'm always exact on my lmp (I think i just ovulate late in my cycle) As long as the fetus has all that it needs to be a viable pregnancy that is all you really need to worry about. Congratulations!!